This commit refactors the reference frame structure used in the
dynamic motion vector referencing system, and makes it support
the bi-directional reference frames. This resolves unit test
failure (enc/dec mismatch) when both are turned on.

The compression performance (ref-mv + ext-refs) is improved by
0.2% for lowres.

Change-Id: I233624d8fccc1f69e82295f94de984ff056365dc